Increasing lower engine emission standards call for increasingly more sophisticated engine controls. On way to improve engine operation is to install pressure sensors in engine cylinders. The pressure sensors may provide useful feedback information that may be indicative of engine combustion for combustion location, combustion amount, quality, engine performance, durability and engine emissions for each of the cylinders that a pressure sensor is installed in and the engine itself. A pressure sensor may be installed in each engine cylinder so that a controller may evaluate the way the cylinder is operating. For example, if any of the mass fraction burn locations for an individual cylinder is delayed longer than is desired, engine fuel injection timing of that cylinder may be advanced to advance the crankshaft location of the mass fraction burn location during an engine cycle for the particular cylinder.
In some engine systems, engine costs and computational power requirements for processing cylinder pressure sensor data may be reduced by relying on a single pressure sensor. For example, Fulton et al. disclose an engine system in US2017/0051700 wherein a single pressure sensor is coupled a single engine cylinder that provides the lowest root mean square error values. The cylinder pressure (e.g., the maximum in-cylinder pressure) for the single engine cylinder is measured via the single sensor, while the corresponding pressure values for remaining engine cylinders is inferred based on the measured pressure data using a model and measured engine operating conditions.
However the inventors herein have identified potential issues with the above approach. The modeled in-cylinder pressure values for the remaining engine cylinders may be error prone. For example, the modeled values may significantly deviate from measured cylinder pressure values (such as values obtained if each cylinder were installed with a pressure sensor). This may be largely due to cylinder-to-cylinder compression pressure variation due to the air distribution (or dynamic charge air effects) and compression ratio differences between the instrumented and non-instrumented cylinders. In addition, engines typically hold an engineering margin for peak cylinder pressures between a maximum limit and a calibration target. This engineering margin accounts for engine-to-engine and cylinder-to-cylinder variability. Errors in peak cylinder pressure estimation can result in larger margins to account for engine-to-engine and cylinder-to-cylinder variability. This in turn can limit the peak torque that can be supported by the engine.
The inventors have recognized that in model, the in-cylinder pressure trace can be decomposed into compression pressure and combustion pressure components. While the combustion pressure is related to a combustion event governed by fuel injection quantity and timing, the compression pressure is influenced by the air distribution and the compression ratio. While the combustion pressure reconstruction of the non-instrumented cylinders could be obtained by correcting via a crank shaft oscillation model (that accounts for the cylinder-to-cylinder variation of a combustion event due to fuel injection quantity and timing variation), the compression pressure reconstruction was not achievable by similar means. For example, the compression pressure of the non-instrumented cylinders were assumed to be identical to that of the instrumented cylinder neglecting the air distribution and compression ratio cylinder-to-cylinder variation. However, the actual cylinder-to-cylinder variation of the compression pressure changes with engine operating parameters such as engine speed. The compression pressure of each cylinder has an impact upon following pressure traces including combustion pressure, and eventually on the maximum in-cylinder pressure. Consequently, the model encounters error in predicting the maximum in-cylinder pressure due to cylinder-to-cylinder variation of compression pressure. In one example, if the compression pressure is not accurately known and a lower value is assumed for safety reasons, the amount of boost pressure that can be provided to the engine may be unnecessarily limited, limiting engine torque output. On the other hand, if a pressure sensor was installed in each cylinder to reduce the error, the cost and complexity benefit of reduced component usage would be lost. Thus, it may be difficult to balance cost of pressure determination and accuracy of pressure determination.
In one example, the above issues may be at least partly addressed by a method for an engine comprising: measuring a maximum in-cylinder pressure in a first cylinder via a pressure sensor; and inferring the maximum in-cylinder pressure in a second cylinder based on a difference from the measured maximum cylinder pressure of the first cylinder, the difference determined as a function of each of intake valve closing timing of the second cylinder, and cylinder identity. In this way, cylinder-to-cylinder pressure estimation variations can be reduced without needing pressure sensors to installed in each cylinder.
As an example, a multi-cylinder engine system may have a single pressure installed inside one of the engine cylinders (hereafter referred to as the instrumented or indicated cylinder), while remaining engine cylinders do not include any installed sensors (hereafter referred to as the non-instrumented or non-indicated cylinders). During conditions when cylinder pressure estimation is required, such as for estimating fuel injection, boost limits, etc., an engine controller may use the output from the sensor to estimate the maximum in-cylinder pressure for the instrumented cylinder and to infer the maximum in-cylinder pressure for the non-instrumented cylinders. Specifically, for a given non-instrumented cylinder, the controller may modify the measured in-cylinder pressure of the instrumented cylinder with one or more correction factors. As an example, the controller may apply a factor (e.g., alpha) that compensates for the difference in fuel injection quantity and another factor (e.g., delta) that compensates for the difference in fuel injection timing between the cylinders. Further, the controller may modify the measured in-cylinder pressure of the instrumented cylinder with yet another factor (e.g., beta) that compensates for the difference in air charge between cylinders based on the difference in compression pressure between the cylinders at intake valve closing (IVC). For a given cylinder, beta may be mapped as a function of engine speed and torque set-point. In addition, beta may be mapped as a function of the location and identity of a given non-instrumented cylinder. This is because even for two non-instrumented cylinders having the same IVC, there may be differences in compression pressure due to differences in RAM air effect. In one example, the factor beta may be mapped during dyno testing or calibration of other engine parameters. Based on the mapped maximum in-cylinder pressure of the non-indicated cylinders and the indicated cylinder, engine operating conditions may be adjusted. For example, a maximum permissible boost pressure may be adjusted to be at or within the maximum in-cylinder pressure. As another example, EGR flow to the engine may be adjusted based on the maximum in-cylinder pressure.
In this way, the accuracy of maximum in-cylinder pressure estimation can be improved without requiring additional pressure sensors to be installed in each cylinder. The technical effect of using a correction factor that compensates for differences in compression pressure between cylinders is that cylinder-to-cylinder air charge estimation variation can be reduced. By more accurately estimating the maximum in-cylinder pressure of each cylinder, the boost pressure that can be provided to the engine can be more accurately determined. For example, a higher boost pressure can be provided to the engine. Likewise, a higher EGR flow may be provided. By accurately estimating an increased maximum in-cylinder pressure, an increased maximum torque output of the engine can be better supported.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.